Currently, 3rd generation (3G) cellular communication systems are being developed to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology. Carrier frequencies are used for both uplink transmissions, i.e. transmissions from a mobile wireless communication unit (often referred to as wireless subscriber communication unit or user equipment in 3rd generation systems) to the communication infrastructure via a wireless serving base station (often referred to as a Node-B in 3rd generation systems) and downlink transmissions, i.e. transmissions from the communication infrastructure to the mobile wireless communication unit via a wireless serving base station (e.g. Node-B). A further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of Universal Mobile Telecommunication System (UMTS), can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
CDMA communication, as used in 3G mobile communications air interface technologies, is an ‘interference limited’ technology from a data throughput perspective. CDMA technology utilises orthogonal variable spreading factor (OVSF) codes combined with pseudo noise (Pn) codes to differentiate multiple UEs that are utilising the same spectrum at the same time for uplink access on the Uu radio interface. In order to maintain sufficient signal-to-interference ratio (SIR) protection for all UEs on accessing the Node-B, up-link (UL) power control (PC) is dynamically managed by the network infrastructure. SIR estimation is commonly derived from pilot tones in the uplink (UL) dedicated paging control channel (DPCCH). User equipment (UE) devices transmitting to a Node-B on the same spreading factor (SF) would be arranged such that their respective transmissions have substantially the same power when received at the receiving Node-B. Often, up to ninety six UEs are simultaneously supported in call mode for a specific Node-B.
Referring now to FIG. 1, a known cellular-based communication system 100 is shown in outline. The cellular-based communication system 100 is compliant with, and contains network elements capable of operating over, an universal mobile telecommunication system (UMTS) air-interface.
A plurality of wireless subscriber communication units/terminals (or user equipment (UE) in UMTS nomenclature) 105 communicate over radio links with a plurality of base transceiver stations, referred to under UMTS terminology as Node-Bs, 115 supporting communication coverage over a particular communication cell 110.
The wireless communication system, sometimes referred to as a Network Operator's Network Domain, is connected to an external network 140, for example the Internet. The Network Operator's Network Domain includes:
(i) A core network, namely at least one Gateway General Packet Radio System (GPRS) Support Node (GGSN) 125 and at least one Serving GPRS Support Node (SGSN) 130; and
(ii) An access network, comprising a UMTS Radio network controller (RNC) 120; and at least one UMTS Node-B 115, where each RNC 120 may control one or more Node-Bs 115.
The GGSN 125 or SGSN 130 is responsible for UMTS interfacing with a Public network, for example a Public Switched Data Network (PSDN) (such as the Internet) 140 or a Public Switched Telephone Network (PSTN). The SGSN 130 performs a routing and tunnelling function for traffic, whilst a GGSN 125 links to external packet networks. Each SGSN 130 provides a gateway to the external network 140. The Operations and Management Centre (OMC) is operably connected to RNCs 120 and Node-Bs 115. The OMC comprises processing functions and logic functionality in order to administer and manage sections of the cellular communication system 100, as is understood by those skilled in the art.
The Node-Bs 115 are connected to external networks, through Radio Network Controller (RNC) stations, including RNC 120 and mobile switching centres (MSCs), such as SGSN 130. The function of the Node-B 115 is to communicate with the RNC 120 and perform chip rate processing on the data signals. A cellular communication system will typically have a large number of such infrastructure elements where, for clarity purposes, only a limited number are shown in FIG. 1.
Referring now to FIG. 2, a known antenna station is illustrated. The antenna station comprises a Node-B (base station) 115 located in a building enclosure 202 to protect the Node-B circuitry from the environment/elements. Each Node-B 115 contains one or more transceiver units and communicates with the rest of the cell-based system infrastructure via an Iub interface (not shown), as defined in the UMTS specification. Each Node-B 115 is operably coupled to an antenna mast 117 for transmitting and receiving signals to/from remote UEs 105, where each antenna mast 117 comprises one or more antenna(e) that may be in a form of an antenna array 119. The Node-B 115 includes rake receivers and modulation/demodulation circuits for encoding transmit signals and decoding received signals. The Node-B 115 also includes clock reference generation to assist the chip rate processing as well as for use in the baseband (BB) and radio frequency (RF) subsystems. RF subsystem functionality of the Node-B provides analogue-to-digital conversion and digital-to-analogue conversion of the receive and transmit signals.
Existing beam forming techniques utilise a single antenna feed from the Node-B 115 to the antenna (or antenna array) 119 and use passive phase shifters and power splitters to generate fixed weightings on each antenna of the antenna arrays.
It is known that some legacy Node-Bs 115 may also provide some control signals to the tower top antenna 119. These control signals are generated centrally by the OMC (illustrated in FIG. 1) and provide for limited electro-mechanical control of the antenna 119, for example mechanical tilt actuation of the antenna 119, controlling gain in tower top receive low noise amplifiers (LNAs), etc.
In the field of wireless mobile communications, replacement of Network infrastructure components and elements, such as specifically a replacement of Node-B (base station) or elements thereof, requires significant engineering effort. In particular, significant effort and testing of the installed components or elements, as well as the impact on the other infrastructure elements that the replacement equipment is connected to, is required to ensure that the operational performance due to the integration of the upgraded equipment is optimised. For example, for a replacement Node-B, such testing would include assessing changes in the Network Operations and Management Centre (OMC), Node-B baseband control plus data interface as well as the impact on the Node-B's radio frequency (RF) components and performance as well as any effect on the antennae or antenna arrays used. Thus, the upgrade requires a co-ordinated effort between infrastructure vendors and Network Operators, and all the associated costs. Thus, replacement of equipment or components is not always feasible.
Referring back to FIG. 2 illustrates a Node-B enclosure 102 that is located adjacent an antenna mast 117 that comprises the antenna array 119 located substantially near or at the top of the antenna mast 117. The Node-B 115 communicates with the antenna array 119 via fixed cabling in a form of co-axial cables 114.
The losses in RF signal strength of this cabling are typically in the order of 6 dB. Hence, for a 20 Watt (+43 dBm) effective radiated power (ERP), a 100 W (+50 dBm) RF power amplifier (PA) would be required.
In order to mitigate the problems associated with cabling power losses, remote radio head solutions are available. Remote radio heads contain RF circuitry including amplification, filtering, RF up-conversion/RF down-conversion, analogue-to-digital conversion/digital-to-analog conversion and use a digital baseband (BB) data interface over a fixed fibre optic link to communicate to/from the baseband devices and integrated circuits (ICs) of the Node-B (base-station). Thus, with remote radio head solutions, a significant amount of processing, together with the RF circuitry is moved to the antenna mast. OBSAI RP3-01 or CPRI BB-RF serial interface standards can be used for such communications.
Node-B baseband communications to the remote radio head need to be configured and integrated with the Node-B baseband, thereby requiring significant associated engineering effort. Furthermore, OMC configuration of such devices needs to be accommodated.
Remote Radio Heads are located close to the antennas on the antenna mast/tower. These solutions are not integrated with the antenna. Thus, some losses still exist in connecting to the antenna elements via a duplexer and passive beam-former unit.
These losses associated with cabling can be reduced compared to a tower base solution. However, the losses are still typically of the order of 3 dB. Hence, for a 20 Watt (+43 dBm) effective radiated power (ERP), a 40 W (+47 dBm) RF power amplifier (PA) would be required. Nevertheless, even 40 W PAs effectively still operate at excessive and very high power levels.
However, remote radio head solutions are physically large and, by necessity due to the PA power and complexity with regard to thermal management of such PAs. High Power PAs also require mechanically large cavity duplexer devices to support the RF power handling capability.
A further problem in the use of remote radio heads results from the usual essential requirement to perform strict thermal management of high power RF PAs. The locating of remote radio heads at the top of antenna masts precludes the opportunity to use climate control systems that are typically used in tower-base buildings. Hence, the reliability of such remote radio head solutions can be compromised as a result, coupled with the limited serviceability aspects of tower-top solutions.
Modern modulation schemes used in many cellular communication systems use high peak-to-average ratios. A peak-to-average ratio of 10.5 dB is not uncommon in many versions of 3rd generation partnership project (3GPP) wireless communication systems, such as: EDGE, wideband code division multiple access (WCDMA), WiMAX and long term evolution (LTE). Therefore, the PA needs to be operating in a linear mode when using these modulation schemes, thereby driving down the PA efficiency to sub 10%. This implies that a 100 W PA consumes in excess of 1 kW DC power.
Major efforts have been underway in recent years to improve this poor power efficiency by utilising schemes such as adaptive predistortion. Predistortion schemes utilise feedback paths where the PA output is monitored and the resultant modulation signal and the distortion detection enables an ‘anti-distortion’ co-efficient to be applied to the (forward path) modulation signal, thereby compensating for (off-setting) the subsequent signal distortion created by the PA. In this manner, the use of predistortion schemes allows the PA to operate in a more non-linear mode of operation, thereby increasing the PAs overall efficiency. Thus, as a result of this efficiency drive, the selection and operation of the PA is closely coupled to the operation of the modulator components.
Thus, although offering some improvement in being able to use less power RF PAs, remote radio heads have struggled to gain acceptance in the wireless communications marketplace. A primary objection to the use of remote radio heads is that their large physical size, when placed atop antenna mast assemblies, has caused a negative public reaction.
Furthermore, mast top installation of such devices, depending upon jurisdiction, is subject to regulatory health and safety enforcements limiting their weight, often to less than 26 kg.
Mast top locations are also generally at a premium for the location of antenna units; thus non-antenna installations can not be given valuable real-estate positions.
Conventional antenna arrays, comprising multiple antenna elements and used with existing Node-B equipment in most 3G installations, utilise a fixed +/−65° beam pattern. Outside of the main lobe of the antenna beam the signals are spatially filtered and significantly attenuated. Conventional network planning and passive antenna array solutions process all incoming signals with a common fixed beam pattern. Such receive processing, based on signals received within the geographic area identified by the antenna beam main lobe, tends to dictate a corresponding common beam pattern for transmitter operation. Thus, an identical radio frequency (RF) footprint is used for both receive (Rx) and transmit (Tx) operation.
Rx beam-forming using antenna arrays depends on the ability to constructively add incident signals on each of the antenna elements in a way that coherently adds those from the desired direction. Thus, incident signals that are not from the desired direction will be incoherently added, and thus will not experience the same processing gain. The term ‘coherency’ implies that the signals will have substantially the same phase angle.
In addition, thermal noise from multiple sources also exhibits incoherent properties, and thus when added the signals from multiple sources do not experience the same processing gain as a coherent desired signal.
Conversely in Tx active antenna arrays the signals are coherently combined within the intended beam pattern as electromagnetic (EM) signals in the ‘air’ so that they arrive coherently at the mobile station (MS) (e.g. UE) receiver.
In a Node-B antenna array arrangement, the received RF signal from a single UE cannot be discerned without demodulation of the composite signal. Individual receive beam-forming for a specific user is not feasible, since there is likely to be multiple received signals of the same power from different UEs simultaneously at the antenna array. Even if few UEs are utilising the Node-B, the likelihood is that the signals would be below the noise floor of the Node-B's receiver. The processing gain of a WCDMA receiver implies that the signal can be extracted from the noise floor. This, however, requires at least a partial demodulation process.
U.S. Pat. No. 5,987,037 describes an antenna array arrangement that deals with transmit-only beam generation at a tower top. The application employed in U.S. Pat. No. 5,987,037 only uses frequency division multiplexed (FDM) signals where mobile access is implemented via one channel per user. Thus, a single modulator is used and the signal is modified on each of the antenna element paths to create the beam pattern. Hence, U.S. Pat. No. 5,987,037 discloses a narrowband carrier allocation per start of call mechanism, where the narrowband carrier can be beam-formed to individual mobile communication units.
U.S. Pat. No. 6,701,137 discloses an antenna system architecture that utilises a digital back-haul link to the BB processing unit. In the architecture proposed in U.S. Pat. No. 6,701,137, beam-forming and digital signal processing are performed in the antenna tower base equipment.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting antenna array technology in a wireless communication network would be advantageous.